Antenna coupling device for coupling an antenna of a hand-portable telephone to a remotely located antenna

ABSTRACT

An antenna coupling device is disclosed for connecting, substantially inductively, a first antenna to a second antenna, whereat the antenna coupling device comprises a loop antenna (4) with two poles (15, 16). The loop antenna (4) is arranged to surround the first antenna and is connected to the second antenna via both of its poles (15, 16). Such an antenna coupling device connected via a cable (5) to an external antenna by the action of threading the antenna coupling device onto a helical antenna (2) of a hand-portable telephone (1) is used inside a vehicle for improving transmission and reception conditions of the hand-portable telephone (1).

BACKGROUND OF THE INVENTION

The invention relates to an antenna coupling device for coupling,substantially inductively, a first antenna, e.g., situated in a vehicleand, e.g., incorporated in a hand-portable telephone, to a secondantenna.

Specifically, the invention relates to an antenna coupling device forcoupling inductively a first antenna, situated in a screened place, to asecond antenna, working under better transmission and receptionconditions. Such an antenna coupling device may be used, e.g., whentravelling in a vehicle for applying onto a helical antenna of ahand-portable telephone, which is not provided with means for agalvanical connection to an external antenna. The telephone may herebybe coupled inductively via an RF coupling means and a coaxial cable toan external antenna for achieving better antenna performance.

An antenna coupling device of this kind is disclosed in U.S. Pat. No. 4220 955, which describes a device for coupling inductively an antenna,which is incorporated in a "transceiver", to an external antenna. Thecoupling device comprises a shielded housing intended to be threadedonto the antenna and containing an helical antenna in the form of a coilfor the inductive coupling.

An isolated sleeve with an open end carries the coil, the one end ofwhich is connected to the center conductor of a transmission line, whichleads to an external device, and the second end of which is open. Thecoil and the sleeve are surrounded by a conductive housing, which iscoupled to the shield of the transmission line.

The above antenna coupling device is distinguished by the use of ahelical antenna, i.e. a coil with one open end, for the inductivecoupling. Further, the diameter of the coil is small in relation to thetransferred wavelengths. This means that the helical antenna isfunctioning in "normal mode". Such is also the case for the helicalantenna built in the "transceiver". Hereby the short range radiationfields of the electromagnetic radiation of the two antennas correspond,which is one of the conditions for satisfactory inductive coupling, whenthe antennas have the same geometrical orientation and are situatedclose to each other.

In order to achieve satisfactory inductive coupling in this prior artantenna coupling device, the helical antenna must also be provided witha relatively large number of turns, in respect to its required diameterand total wire length, and the turns must be distributed spaciously,i.e. the helical antenna shall have a large axial elongation.

If the helical antenna is to be compressed axially through a morecompact winding this may to some extent be compensated by a greaternumber of turns, which requires a more complicated design and createsdifficulties in achieving the same efficiency.

SUMMARY OF THE INVENTION

The object of the invention is to achieve an antenna coupling devicethat overcomes as far as possible the above mentioned drawbacks and thatfulfills the demands of a high degree of coupling, simple and compactdesign, and uncomplicated operating.

A loop antenna has other features than a helical antenna. The loopantenna works by both its ends, or poles, being galvanically coupled.Hereby it presents another radiation pattern or field. However, in theimmediate proximity of antennas of the different types, i.e. within theshort range fields, the radiation patterns are approximately the same.This enables a loop antenna to be designed to give excellent inductivecoupling to a helical antenna, provided that these have the samegeometrical orientation and that the loop antenna preferably surroundsthe helical antenna.

Further, it is advantageous to use a loop antenna, since it gives a highdegree of efficiency for the inductive coupling with a very simpledesign. An electrically well-dimensioned loop antenna including awell-tuned impedance matching unit may in one single turn give a degreeof efficiency of 50-70%. Hereby the loop antenna may be given anexceptionally small size and an easily adapted design. With thissolution it is also possible to achieve an extremely broad-bandedantenna function.

It is also possible to provide the loop antenna with more than one turn.This way one could, e.g., achieve an impedance matching to a cablewithout the use of separate reactive elements or tuned conductorelements in connection with the loop antenna.

Thus, the object of the invention is achieved by an antenna couplingdevice of the outlined type, said antenna coupling device comprising aloop antenna with two poles, the loop antenna being arranged to surrounda first antenna, and the loop antenna being connected, via both itspoles, to the second antenna.

It is advantageous that the loop antenna is connected to the secondantenna via an impedance matching unit, which is coupled directly to theloop antenna. Hereby the impedance of the loop antenna may easily beadapted to an arbitrary impedance, irrespective of the diameter and thenumber of turns of the loop antenna and irrespective of the frequencyrange used for transmitting and receiving.

It is also possible to connect the impedance matching unit via a cableat some distance from the loop antenna. However, this decreases thedegree of efficiency as a result of a high standing wave ratio in thecable.

Further, it is suitable to connect the loop antenna to the secondantenna via a transmission line in order to freely place the antennacoupling device and the second antenna, respectively, in suitablepositions. For example, the antenna coupling device may for this purposebe made easily accessible inside a vehicle for a user of a hand-portabletelephone, whereat the second antenna may be mounted externally on thevehicle body.

In this case, the impedance matching unit adapts the loop antenna to theimpedance of the transmission line, so that the least possible lossesoccur and the highest possible degree of efficiency is achieved. It isadvantageous to choose a coaxial cable as the transmission line, sinceit is well-suited for transferring RF signals.

In a preferred embodiment of the invention, the second antenna is amonopole or dipole antenna, which is in principle an RF radiating,straight conductor, or a co-linear antenna, constituted by two or moremonopoles with a coil arranged between adjacent monopoles.

In order to achieve satisfactory coupling, or high degree of efficiency,the loop antenna is arranged to surround or enclose the first antennasubstantially coaxially with a small mutual radial separation.

In the first preferred embodiment, the loop antenna and the impedancematching unit are arranged within a housing that may be threaded ontothe first antenna and that arranges the loop antenna substantiallycoaxially to the first antenna. Preferably, the housing isnon-conductive, although it may be made of conductive or metallizedmaterial, whereby it may be connected to the shield of the coaxialcable.

The housing is provided with at least one mechanical fixing means forfastening onto the first antenna. The mechanical fixing means mayinterlock with a groove or a shoulder, be clamped or screwed either ontothe first antenna or onto the hand-portable telephone itself.

Another important advantage in the use of a loop antenna is that it mayeasily be arranged on a printed circuit board together with theimpedance matching unit.

Further advantageous features of the invention are described in thedependent claims.

The invention allows several modifications without departing from itsmain principles. For example, the antenna coupling device may be usedfor coupling inductively a first antenna to any type of RF means.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described closer below in a preferred embodiment withreferences to the accompanying drawings, wherein:

FIG. 1 shows, in a partly cut away side view, a hand-portable telephoneprovided with a helical antenna, whereon an antenna coupling deviceaccording to the invention is applied;

FIG. 2 shows, in a top view, a principle plan of the main components ofthe antenna coupling device of FIG. 1; and

FIG. 3 shows a suitable way of arranging the antenna coupling device ofFIG. 1 on a printed circuit board.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the preferred embodiment the antenna coupling device is used inside avehicle or a building for coupling inductively a hand-portable telephoneto an external antenna, which hand-portable telephone is not providedwith means for a galvanical connection to an external antenna.

The hand-portable telephone shown in FIG. 1 is provided with two antennameans, one of which is a helical antenna, constituting a first antenna,and the other of which is an extendable and retractable antenna rod. Thehelical antenna is moulded into a substantially cylindrical element 2projecting outward from the chassis 1 of the telephone. FIG. 1 showsonly an upper knob 3 of the antenna rod which is retracted through thehelical antenna.

Further, an antenna coupling device according to the invention is shown.This includes a loop antenna 4 with an impedance matching unit arrangedon a printed circuit board within a housing 6, which is made of anon-conductive shell with a substantially cylindrical opening on theunderside adapted for receiving the substantially cylindrical element 2of the helical antenna. The loop antenna 4, which is fixed in thehousing 6, coaxially with the substantially cylindrical opening, has aminner diameter which is slightly greater than the diameter of thesubstantially cylindrical element 2.

One end of a coaxial cable 5, which extends substantiallyperpendicularly to the axis of the loop antenna 4, is coupled to theloop antenna 4 and the impedance matching unit of the antenna couplingdevice. The other end of the coaxial cable 5 is connected to a secondantenna 20, shown schematically in FIG. 1, mounted externally on thebody of the vehicle, on the outside of the building or similarly.Preferably, the second antenna 20 is a monopole antenna or a colinearantenna.

The principle plan of FIG. 2 shows the electrically active components ofthe antenna coupling device of FIG. 1. The loop antenna is a conductiveloop with two poles (ends) 15, 16 with an optional number of turns,preferably approximately one turn.

Further, an impedance matching unit 7 is shown, which is a quadripolewith passive reactive elements for transformation of impedance. Thepoles 15, 16 of the loop antenna are connected to the one pair of thepoles of the impedance matching unit 7 and the coaxial cable 5 to theother pair.

FIG. 3 shows the printed circuit board 8, on which the loop antenna 4(FIG. 1 and 2) and the impedance matching unit 7 (FIG. 2) are arranged.The printed circuit board has a substantially rectangular shape and isprovided with a circular hole 14, intended for receiving the firstantenna and being offset from the center to the one short side of therectangle. The printed circuit board is somewhat wider around the hole14.

The printed circuit board 8 is provided with a first conductor pattern 9adjacent to the main portion of the periphery of the hole 14, the widthof the first conductor pattern 9 being substantially even andconsiderably smaller than its total length. This conductor patternconstitutes the loop antenna.

At the one end of the first conductor pattern 9 there is provided a hole11 for connecting the shield of the coaxial cable 5 (FIG. 1 and 2). Asecond conductor pattern is arranged between the ends of the firstconductor pattern 9 and is provided with a hole 10 for connecting thecenter conductor of the coaxial cable. In the two spaces 12, 13 presentbetween the ends of the conductor patterns reactive components (notshown) forming the impedance matching unit are mounted and connected.

The printed circuit board 8 is fixed inside the housing 6 (FIG. 1) andits right part is used for fixing the cable.

I claim:
 1. An antenna coupling device for providing RF energy coupling,substantially inductively, between a first antenna included in ahand-portable telephone, and a second antenna remotely located withrespect to the telephone, said antenna coupling device comprising atransmission line having first and second ends, the first end beingconnected to a loop antenna with two poles, said loop antenna beingarranged to surround the first antenna, the second end of saidtransmission line being connected to the second antenna.
 2. The antennacoupling device according to claim 1, wherein the loop antenna isconnected to the first end of the transmission line via an impedancematching unit, which is coupled to the loop antenna.
 3. The antennacoupling device according to claim 2, wherein the impedance matchingunit is arranged so as to match an impedance of the loop antenna to animpedance of the transmission line.
 4. The antenna coupling deviceaccording to claim 2, wherein the loop antenna and the impedancematching unit are arranged inside a housing that is threadedlyconnectable onto the first antenna.
 5. The antenna coupling deviceaccording to claim 4, wherein the housing is provided with at least onemechanical fixing means for fastening on the first antenna.
 6. Theantenna coupling device according to claim 4 or 5, wherein the housingincludes means for aligning said loop antenna substantially coaxiallywith the first antenna.
 7. The antenna coupling device according toclaim 2, wherein the loop antenna and the impedance matching unit arearranged on at least one printed circuit board.
 8. The antenna couplingdevice according to claim 1, wherein the transmission line is a coaxialcable.
 9. The antenna coupling device according to claim 1, wherein theloop antenna is impedance matched through its length.
 10. The antennacoupling device according to claim 1, wherein the loop antenna isarranged for inductive coupling to the first antenna, said first antennabeing a helical antenna.
 11. The antenna coupling device according toclaim 1, wherein the second antenna is a monopole antenna.
 12. Theantenna coupling device according to claim 1, wherein the loop antennasurrounds the first antenna substantially coaxially with a small radialseparation.
 13. The antenna coupling device according to claim 1,wherein the second antenna is a dipole antenna.
 14. The antenna couplingdevice according to claim 1, wherein the second antenna is a colinearantenna.